The primary step in cardiac morphogenesis is the organization of nondifferentiated mesoderm cells into bilaterally distributed precardiac fields, These heart-forming fields continue to develop as they fuse at the embryonic midline to form the primary heart tube-a process that accompanies further cell differentiation to a terminal cardiac phenotype. Several extracellular signaling factors have been implicated for promoting the formation of cardiac tissue in the early embryo. Prominent among these "cardiac promoting" factors are molecules that regulate WNT signal transduction. Data from our laboratory has demonstrated that WNT11 is an essential regulator of early cardiogenesis. However, it has also been shown that Dkk1 and crescent - molecules initially characterized as WNT inhibitors - appear to possess identical cardiac functional activities as WNT11. It is now clear that the reason these molecules share functional similarities is due to the multiplicity of WNT signaling pathways. Our research efforts have helped establish that non-canonical WNT signaling plays a major role in promoting cardiac tissue formation. Moreover, recent data has demonstrated that the similar cardiac inducing activities of WNT11, Dkk1, and crescent are due to their shared ability to activate Jun-N-terminal kinase (JNK). To build on these findings, we plan to examine in detail the functional role of these signals for formation of the heart. Accordingly, four complementary specific aims are proposed that involve examining: 1) whether WNT11/Dkk1/crescent signals promote cardiogenesis by direct action on non-differentiated mesoderm, 2) functional differences among WNT11, Dkk1, and crescent in promoting cardiac differentiation, 3) the relationship between WNT/JNK and WNT/R-catenin pathways in establishing the vertebrate heart, and 4) the involvement of noncanonical WNT signals in promoting cardiac organogenesis in conjunction with FGF and BMP proteins. These proposed studies should increase our understanding of the molecular processes that promote the formation of the heart, and may lead to insights into congenital defects.